Individually driven spindles of a spinning frame or machine can be provided with respective suitable asynchronous motors which are fed from a supply-current source with a supply-current at a frequency which determines the drive speed of the motor and with a voltage which is in a certain ratio to that frequency at the operating speed of the motor.
The term "operating speed" is here used to refer to the speed of the motor and of the spindle during normal spinning operations. This operating or nominal speed is generally constant over the operating duration and deviations from this operating speed generally occur only on shutdown or start-up of the motor or spindle.
After a yarn break has been cleared at an individual spindle which is usually brought to a standstill during the process of clearing the yarn break, it is necessary to accelerate the spindle to its operating speed corresponding to the speed of the remaining operating spindles and, to avoid flaws in the yarn, this acceleration is generally done at a high rate. The duration over which the spindle is accelerated to its operating speed normally is about three seconds and the operating speed usually is between 8,000 and 20,000 RPM.
It is, therefore, important, as a rule, with spinning machines, to accelerate the individual spindles after elimination of a yarn breakage to the normal operating speed in the briefest possible time span.
With textile spindles having whorls which are driven by tangential-belt drives, it has already been proposed to accelerate the spindle to normal operating speed by providing mechanically an enhanced coupling of the whorl to the belt and thus a more efficient transfer of displacement to the whorl by the belt utilizing an increased pressing force of the whorl against the belt or vice versa over at least the greater part of the acceleration period. Such a system is described, for example, in German Open Application DE-OS No. 34 13 764.
Obviously, this technique cannot be utilized where the spindle is substantially directly coupled to an individual motor.